Expand `extract_boot_files` to deal with non-local kernel & initrd files
As a tree-based installation will have to download both kernel & initrd, let's expand
extract_boot_files so those could be downloaded from the internet using the kernel & initrd path from the tree.
This change may or may not be done in the
ISOExtractor class. I do believe creating a new
Extractor class, from which ISOExtractor would inherit (as well as TreeExtractor) would be the way to go, but I'd like to hear from Felipe as well.